Magnetic resonance imaging (MRI) is a well-known medical imaging technique that allows visualizing internal structures of the human (and the animal) body in great detail.
With gradient echo sequences, which are widely used in MRI, the nuclear spins of a sample are excited by less than 90° by means of radiofrequency (RF) pulses. Magnetic gradient fields are then used to dephase and refocus the transverse magnetization of the excited nuclear spins.
A gradient echo sequence which is often used for rapid imaging purposes is balanced steady-state free precession (balanced SSFP). With balanced SSFP which is also known under the acronym TrueFISP (for ‘true fast imaging with steady-state precession’), the magnetization is allowed to accumulate a constant phase within every repetition time (TR) interval by fully rephasing the transverse magnetization between successive RF pulses, i.e. within every TR. As a result, balanced SSFP shows a mixed T2/T1-weighting offering a bright fluid signal and an overall improved signal-to-noise ratio (SNR) in comparison to incoherent SSFP pulse sequences, such as spoiled gradient echo (SPGR). Over the last two decades, balanced SSFP imaging has become increasingly popular for applications where rapid data acquisition is needed, e.g., for cardiac imaging, native magnetic resonance angiography, or thoracic imaging, but remains challenging even on current state-of-the-art MRI systems, since for constant phase accruals, all gradient moments have to be perfectly balanced within every TR. As a result, balanced SSFP imaging is prone to any source of imperfection that perturbs the perfectly balanced gradient scheme, such as eddy-currents.
Similarly, balanced SSFP imaging shows a pronounced sensitivity to off-resonances leading to a periodic modulation of the steady state having high intensity signal regions, frequently referred to as ‘pass-band’ regions, and so-called ‘stop-bands’ or ‘banding artifacts’, where the signal comes close to zero. Hence, imaging of tissues with high susceptibility variations (e.g., lung parenchyma, bone tissue, cartilage), becomes challenging and requires proper shimming to achieve an excellent main magnetic field homogeneity. Alternatively, banding can be effectively mitigated by shortening the TR and has become available only with the introduction of very fast, strong and precise gradient systems. Recently, it has been demonstrated that the TR of Cartesian balanced SSFP imaging can be pushed close to about 1 ms on a typical whole-body MRI system, providing artifact-free chest imaging at 1.5 T (Bieri O. Ultra-fast steady state free precession and its application to in vivo (1) H morphological and functional lung imaging at 1.5 Tesla. Magn Reson Med. 2013 Jun. 28. doi: 10.1002/mrm.24858). One of the major limitations of contemporary Cartesian sampling schemes, however, is its inherent sensitivity to motion, resulting in shifted copies (“ghosting artifacts”) of the object along the phase-encoding direction.
A possible strategy to mitigate motion sensitivity, is to use non-Cartesian encoding schemes, such as the acquisition of k-space data in radial directions (Crémillieux Y, Briguet A, Deguin A. Projection-reconstruction methods: fast imaging sequences and data processing. Magn Reson Med 1994; 32:23-32, Rasche V, de Boer R W, Holz D, Proksa R. Continuous radial data acquisition for dynamic MRI. Magn Reson Med. 1995 November; 34(5):754-61). With radial acquisition, the data is acquired along a plurality of rotated radial spokes in k-space. Here, the k-space center is acquired at every TR and overlapping radial spokes have a strong motion averaging effect. The remaining signal energy produced by the patient's motion spreads over the image in form of streaking artifacts radiating from the motion-affected region, which usually only has a mild effect on the image quality. In addition, due to the frequent sampling of the k-space center, radial imaging is also well suited for time-resolved reconstruction (Barger A V, Block W F, Toropov Y, Grist T M, Mistretta C A. Time-resolved contrast-enhanced imaging with isotropic resolution and broad coverage using an undersampled 3D projection trajectory. Magn Reson Med 2002; 48:297-305, Winkelmann S, Schaeffter T, Koehler T, Eggers H, Doessel O. An optimal radial profile order based on the Golden Ratio for time-resolved MRI. IEEE Trans Med Imaging. 2007 January; 26(1):68-76, Bauman G, Johnson K M, Bell L C, Velikina J V, Samsonov A A, Nagle S K, Fain S B. Three-dimensional pulmonary perfusion MRI with radial ultrashort echo time and spatial-temporal constrained reconstruction. Magn Reson Med. 2015 February; 73(2):555-64).
Similar to Cartesian MRI, the sampling density of a fully sampled radial dataset is determined by the Nyquist limit and undersampling will thus results in streaking artifacts in the image. However, due to spatial incoherences of the undersampling artifacts, radial imaging is well suited for the application of an advanced reconstruction approach such as compressed sensing (Candes E J. Compressive sampling. Madrid, Spain: Intl Congress of Mathematicians; 2006, Lustig M, Donoho D, Pauly J M. Sparse MRI: the application of compressed sensing for rapid MR imaging. Magn Reson Med 2007; 58:1182-1195). Nevertheless, radial trajectories have higher sampling requirements than Cartesian methods as a result of a less efficient k-space coverage. This becomes especially critical for three-dimensional (3D) imaging, where radial imaging leads to prolonged acquisition times. Here, an interesting alternative represents the use of a 3D stack-of-stars sampling scheme (Peters D C, Korosec F R, Grist T M, Block W F, Holden J E, Vigen K K, Mistretta C A. Undersampled projection reconstruction applied to MR angiography. Magn Reson Med. 2000 January; 43(1):91-101, Wu Y, Korosec F R, Mistretta C A, and Wieben O. CE-MRA of the lower extremities using HYPR stack-of-stars. J Magn Reson Imaging 2009; 29:917-923). This approach combines two-dimensional (2D) radial sampling in the plane and Cartesian encoding along the third dimension, which is more time-efficient than a full 3D radial coverage, i.e. the acquisition of a 3D star of data formed by radial spokes in k-space.
Although conceptually, radial data acquisition is known since the early days of MRI, its application in the clinical setting is still limited due to technical challenges. A major issue represents proper data reconstruction in the presence of even tiny deviations from the nominal k-space trajectory, e.g., caused by gradient system imperfections, such as eddy currents or heating effects and internal synchronization errors. In general, any discrepancy between the nominal and actual k-space trajectory results in data inconsistencies, i.e., perpendicular and parallel offset of the central k-space point with respect to the k-space center emanating in the image as increased diffuse background noise (‘smearing’ artifacts or ‘halo-effect’). This effect becomes especially prominent for half-echo centered-out acquisition schemes, such as ultra-short echo time (UTE) SPGR sequences (Johnson K M, Fain S B, Schiebler M L, Nagle S. Optimized 3D ultrashort echo time pulmonary MRI. Magn Reson Med 2013; 70:1241-1250, Togao O, Tsuji R, Ohno Y, Dimitrov I, Takahashi M. Ultrashort echo time (UTE) MRI of the lung: assessment of tissue density in the lung parenchyma. Magn Reson Med 2010; 64:1491-1498), where data sampling starts directly in the k-space center, such that data are acquired in radial directions from the center towards the periphery of k-space. For UTE sequences the major methodological advance of the half-echo acquisition scheme relates to the substantial shortening of the echo time (TE), whereas for balanced SSFP it would offer a shortened TR and thus a reduction of banding artifacts. Unfortunately, centered-out half-radial sampling in combination with balanced SSFP imaging is particularly affected by the aforementioned MRI-system imperfections.